In recent years, “printed electronics”, which is a technique for forming a circuit (patterning) by printing on a resin substrate having particularly high flexibility, is attracting attention as the next-generation industrial infrastructure replacing the semiconductor industry. This technique is applicable to from basic circuit components such as a thin-film transistor, a resistor, an inductor, and a capacitor, to a cell, a display, a sensor, a radio frequency identification (RFID) tag, a solar cell, etc. The technique is expected to dramatically simplify manufacturing processes of electronic products or the like, shorten the manufacturing time, and simultaneously achieve savings in energy and resources.
Personal digital assistants among electronic products have remarkably spread, and enhancement in the performance of the devices have led to increases in the size and weight thereof. Accordingly, reducing the weight of personal digital assistants is strongly demanded in the market. Moreover, since the thickness of personal digital assistants has become thin in response to the demand for weight reduction and quality design, glass used for the display, the sensor or the like are easily damaged when dropped. Thus improvement in durability has become an issue. To solve the problems, constituent parts of the personal digital assistants have been reviewed and material with high impact resistance has increasingly been adopted. However, no fundamental solution has yet been found. Consequently, a lightweight and flexible polymer film or the like has been promoted to be used in place of conventional printed circuit substrates and glass substrates among materials which constitute personal digital assistants.
In order to use a polymer film substrate for personal digital assistants, an electrical wiring is required to be formed on the polymer film surface. The electric wiring pattern formed on a polymer film surface is required to have high conductivity, adhesion to a substrate, and flexibility, and also be able to form a fine circuit pattern.
Generally, methods for forming a conductive circuit pattern on a polymer film substrate include an electroplating method, an electroless plating method, a vapor deposition method such as sputtering, and a method in which a conductive paste is applied. However, the electroplating method and the electroless plating method, in which a large amount of solvent is used, are costly in terms of waste liquid treatment. The vapor deposition method such as sputtering is also costly not only because a large-scale apparatus is required but also because etching for removal of unnecessary parts is required after vapor deposition onto the entire substrate.
Other methods for forming a circuit pattern include a method in which a conductive ink or paste of the baking type or the thermosetting type is used. In the method using a baking-type ink or paste, conductivity is imparted by removing a solvent and a binder component by heating generally at a high temperature of 200° C. or more to sinter conductive particles or bring them into contact with each other. In the method using a thermosetting-type conductive ink or paste, conductivity is imparted by curing a composition containing conductive particles and a thermosetting resin as a binder component to bring the conductive particles in the cured material into contact with each other.
In the method using a baking-type ink or paste, since a conductive particle component exists alone in the cured material, low resistivity can be obtained. However, there is no chemical bond at the interface between the cured material and a substrate, and adhesion to the substrate is likely to be low. In particular, when a circuit pattern is formed on a substrate comprising materials with low surface tension such as indium tin oxide (ITO), adhesion is significantly low.
As a material of the substrate, a multilayer substrate obtained by a build-up method is used for a lighter weight than conventional printed circuit substrates. However, a build-up method requires complicated processes. A flexible printed circuit substrate is light in weight, superior in flexibility, and also excellent in heat resistance, but is costly because polyimide is used as a material thereof. Polyamide and polyester are examples of cheaper film materials, but they are thermoplastic resins with low heat resistance. Therefore a conventional conductive paste, which needs high temperature treatment, is difficult to be used thereon.
A conductive circuit may also be formed directly on a glass surface of apart such as a display, which requires transparency and good design. Glass is excellent in heat resistance, but not satisfactory in weight, flexibility, and impact resistance. Although there seems to be a demand for a transparent plastic material like polymethylmethacrylate in place of glass for a part requiring transparency and good design, a transparent plastic material cannot replace glass in terms of heat resistance as previously mentioned.